To optimise diesel engine combustion, it is necessary to have precise control over the quantities of fuel delivered by the fuel injectors. It is desirable to be able to inject small quantities of fuel across a wide range of fuel pressures. For heavy-duty applications in particular, the fuel injectors must be capable of delivering fuel in small quantities at very high fuel pressures.
Typically, a fuel injector includes an injection nozzle having a nozzle needle which is movable towards and away from a nozzle needle seating so as to control fuel injection into the engine. The nozzle needle is controlled by means of a nozzle control valve (NCV), which controls fuel pressure in a control chamber for the nozzle needle.
Small and controllable injection quantities can be achieved by reducing the opening rate of the valve needle during the needle-opening phase, whilst maintaining a high closing rate during the needle-closing phase. One way of achieving an asymmetric opening and closing characteristic is to modify the NCV to define a restricted flow path for fuel flow between the control chamber and a low pressure drain, as described in WO 2004/005702.
A further requirement of the needle-opening phase is that movement of the valve needle is not hindered unduly by the effect of Bernoulli forces as the valve needle lifts away from the nozzle needle seating. It is also important that the needle does not approach its lift stop at such a high speed that it suffers needle bounce, and that the rate of movement of the valve needle during the intermediate stages of lift is within desired limits.
It is an object of the invention to provide a fuel injector which achieves the aforementioned requirements of the needle-opening phase.